Nonlinear Extensional Rheology of Poly(n-alkyl methacrylate) Melts with a Fixed Number of Kuhn Segments and Entanglements per Chain

Molecular theories for dynamics of entangled polymers are based on both the number of Kuhn segments per entanglement N-e and the number of entanglements per chain N/N-e. Extensive studies have shown that, for polymer chains in the solutions or melts, linear viscoelasticity can be properly normalized, whereas the nonlinear extensional rheological properties cannot be normalized when N/N-e is kept the same. The failure of the latter normalization has been attributed to a difference in N-e. Nevertheless, nonlinear rheological studies are lacking for a suitable model system with fixed N-e and N/ N-e. In this study, we identify poly(n-alkyl methacrylate)s with the number of carbons per alkyl group below seven as a model system. We find that the degree of the transient strain hardening during extensional flow strengthens with increasing the size of the alkyl group even when N-e and N/N-e are kept the same, which is attributable to the weaker friction reduction when the main backbones are more separated.

Automated summary

Molecular theories for dynamics of entangled polymers are based on the number of Kuhn segments per entanglement and the number of entanglements per chain. Linear viscoelasticity can be properly normalized, but normalization fails in the nonlinear extensional rheological properties. This study identifies a suitable model system with fixed parameters and finds that the degree of transient strain hardening during extensional flow increases with alkyl group size, attributed to weaker friction reduction when the main backbones are more separated.